The human eye can cope with a wide range of luminances but cannot cope with the whole range all at once. For example car headlights by day look dim, but can dazzle a driver at night. Similarly, moving from a dark room into sunlight (or vice-a-versa) can make objects in the room difficult to see for a while. This phenomenon of luminance adaptation is also known as discomfort glare or disability glare.

Similarly, the ability to see detail on a computer screen or other display screens relies on the contrast of the information being displayed and it's background. If the eye has been looking at very bright areas of a room then the screen image and background will both appear dark and it will be difficult to resolve the detail. The ideal luminance of the area surrounding a computer video screen should be close to that of the screen display itself. This allows the user to look away from the screen and back again without losing the ability to discriminate between the luminance of the detail on the screen and the background. It is important to consider those surfaces that are likely to be looked at in rapid succession. These are likely to be soruce documents, the keyboard, the display screen, as well as adjacent walls, partitions and other surfaces.

To reduce this luminance imbalance and avoid distracting reflections it is necessary to either eliminate any high luminances from the room and/or to ensure that any high luminances present are not directed or reflected towards the various computer/video display screens.

APPLICATIONS :

• Software Development Centres
• Brokerage houses
• Banks
• Graphics and CAD centres
• Air Traffic Control rooms

THE SOLUTION

If the selected luminance limit angle for the luminaires is too high a number of users in the area will see screen reflections. To balance these risks and ensure safe luminance limits are selected three standard luminaire types have been defined each with a different luminance limit angle. The luminaires so derived are referred to as Category 1, 2, 3 respectively. In general the greater the density of display screens in an area, the greater the intensity of use, the more critical the errors, the lower the luminance limit angle.

These louvers are made from high-purity Aluminium(99.8% or greater) which is anodized and especially brightened to provide superior reflectance. This superior reflectance insures that the total reflectance is greater than 80% and yet the total diffuse reflections is less than 35% .This allows the luminaire manufacturer to control the light and provide better and more focused lighting with minimum glare. The leading manufacturer of this material in the world is Ano-Fol Ltd. (U.K). A chart of some of the louver grade Aluminium they manufacture is shown in Table below.

ANO-FOL Designation	Total Reflectance
	Total (%)	Diffuse(%)
910.31	87.0	3
715.30	86.5	8
730.33	86.5	12
740.33	86.0	18
741.33	85.0	25

CHOOSING THE RIGHT CATEGORY OF LUMINAIRE : A DIGEST

The Chartered Institution of Building Services Engineers (CIBSE) has detailed specifications for Category 1, 2 and 3 type luminaires. An abridged version of these specifications is discussed below.

As mentioned earlier there are three categories listed in CIBSE's LG3. The compliance of a luminaire is achieved by establishing that the calculated average luminance at, or any point above, the specified luminance limit does not exceed the required luminance limit(which is specified as 200cd/m2 by CIBSE's LG3). The specified luminance limit angle is 55 for Category 1, 65 for Category 2 and 75 for Category 3 as shown in the figure.

It must be emphasized that Category 1 is not 'better' than Category 2, nor Category 2 'better' than Category 3. The correct category must be selected for each individual installation otherwise undue installation and energy costs are likely to be incurred.

When choosing the 'right' Category another aspect of the installation is the spacing to height ratio. This ratio must be determined so that the luminance is evenly distributed as can be seen in the figure.

From this information it can be ascertained that in general Category 1 luminates are recommended where there is intensive use and/or a high density of computer , video and other types of display screens. This is particularly relevant if the area has a large open plan or if high definition screens are used. Areas that should consider Category 1 are Brokerage Firms, Computer Aided Design Offices, Air Traffic Control Centres, Specialized Graphics Shops, Ticketing Areas in Travelling agencies and other offices where constant screen interrogation is prevalent.

Category 2 luminaires are recommended where computer/video screens use in less intensive but still fairly widespread. Most offices where computers are used regularly but not very intensively should consider Category 2 luminaires.

Category 3 luminaires are recommended where the density of computer/video screens is low and where computer use is casual and where the use of computers is indicatal to the main activity.

Wide spacing showing uneven distribution.

Optimum spacing showing even distribution.

UPLIGHTING

Another way to light a room is using uplighting. Uplighting means that the light is direct upwards (towards the ceilings, walls and other surrounding surfaces) and the reflected light is used to light the room. As can be seen this is a tricky method and id usually used only when direct lighting cannot be used. Since uplighting depends on surfaces that do not control the light this method is also not very energy efficient. Uplighters can be free standing , furniture mounted, suspended or wall mounted. The luminous intensity distribution of uplighters is often asymmetric (a must for wall mounted uplighters). A sketch of a wall mounted uplighter, the most common type of uplighting used for cove lighting, is shown in the figure.

Uplighters are used when the ceiling is very low(usually between 2.5 and 3.5 metres). Uplighters can only be used with ceilings lower than 2.5m if careful attention is paid to the luminance intensity distribution and menas are taken to avoid the creation of high luminance spots. In order to increase the energy efficiency to uplighters it is desirable to have high reflectance room surfaces with a matt finish and a pale color (preferbly white). When ceilings have exposed structure care should be taken to have as smooth a profile as possible to avoid sharp changes in luminance.

In general, uplighters are 10-20% less efficient than most (downlighting) luminaires however is certain cases there is little alternative, especially when a "wall washing" effect is required.

ELECTRICITY CONSUMPTION AND RETROFITTING

In today's electricity driven buildings almost 40% of the electricity used is for lighting. Other heavy uses of electricity are office equipment and air-conditioning. As air conditioning compressors and other office equipment have become more efficient, in many cases much of the wasted energy in today's buildings is from existing fluorescent lighting fixtures. The poor finish and design of these luminaires scatters as much as 85% of the light as diffused or "uncontrolled light". About 10-15% is absorbed by the Aluminium itself.

Many of these luminaires have non-parabolic profiles, which scatters the light. High school mathematics taeches us that the parabola is the only way to focus a beam of light emanating from the focus of the parabola. Hence putting a light source (tube or bulb) in the focus of a double-parabolic luminaire (where both directions of reflectors are parabolic) is the most energy efficient way of using and controlling light. The heart of a luminaire is the louvre (the box,ballast and various fittings are passive parts).

ELEKTROMAG is the leading manufacturer of double-parabolic louvers in India. In many cases luminaires made using Elektromag's deep-cell double-parabolic louvers can be retrofitted into existing installations. The ideal depth of this luminaire should be 85mm and the light source should be 65mm from the top, so that it can be enveloped by the louver. By ensuring a better controlled distribution of light the original lighting levels can be improved on, with a visually more comfortable light using fewer light sources (tubes,bulbs, etc.,).Energy costa are often reduced to a level, where pay back periods are usually less than 2 years! Few investments have this extent of returns!